Corrosion behavior of new rare-earth free Cu-based metallic glasses in NaCl solution of different molarity

The present study reports the glass-forming ability, density change on crystallization and corrosion behavior of newly developed rare-earth free Cu-based metallic glasses (CBMGs). The critical casting thickness of these new CBMGs increased from 15 mm at 8 at.% Hf to 22 mm at 12 at.% Hf, reaching a maximum value of 28.5 mm at 13.5 at.% Hf. It dropped to 24 mm and 15 mm thereafter, for 15 and 20 at.% Hf, respectively. The alloys showed a monotonic increase in density going from 8 at.% Hf to 20 at.% Hf, with the density of amorphous samples increasing from 7.83 g/cm3 to 8.84 g/cm3 and crystallized samples increasing from 7.92 g/cm3 to 8.91 g/cm3. Potentiodynamic polarization tests carried out in 0.1, 0.2 and 0.5 M NaCl solution at 25 °C showed a direct correlation between corrosion resistance and Hf content in the order: 8 at.% < 13.5 at.% < 20 at.% Hf. Increase in NaCl concentration also had a significant impact on the corrosion intensity due to the chaotropic effect. Scanning electron microscopy of corroded sample surfaces showed formation and eventual merging of individual micro cracks resulting in a mud-cracking pattern. Energy-dispersive X-ray spectroscopy maps of pitted regions revealed reduction in Zr and Al concentration with increase in Hf and Cu percentage for all compositions. An increase in the O concentration along with the appearance of new peaks in the X-ray photoelectron spectroscopy results, measured post corrosion, confirmed the formation of metal oxides and oxyhydroxides in the passive layer.